Powder coating technology has generally evolved over several years into several different coating techniques performed with various types of apparatus. With each technique and apparatus, a powder, such as a resinous polymer or paint, is initially adhered to a desired object or part generally in a uniform thickness. The initial powder coating is then cured using heat or other techniques, such as infrared or ultraviolet light, to fully adhere the coating to the part.
Conventional techniques for initially adhering the powder particles to the part before curing have included three general types. Two general techniques involve the use of fluidized powder beds. In the first technique, the part is heated and then dipped into a fluidized bed of powder particles. The particles partially coalesce or tackify and thereby stick to the part. The second technique involves electrostatically charging the powder particles eminating in a cloud from a fluidized powder bed. When an electrically conductive, but grounded part is placed within the eminating cloud of electrostatically charged powder particles, the charged particles will be attracted to the outer surfaces of the part. The grounded part may be manually placed within a powder coating hood containing the electrostatically charged powder cloud or may be on a conveyor system which moves parts continuously through the hood or chamber. A third general technique for powder coating parts also involves electrostatic charging of powder particles. However, this technique utilizes a spray gun which may emit the powder particles from the gun while electrostatically charging those particles at the gun nozzle or may emit a stream of powder particles using air which has previously been electrostatically charged. In this technique, the parts to be coated are again grounded and are typically contained within a spray coating hood to contain and collect excess sprayed powder. The three general techniques described above may also have many variations.
Each of the above described powder coating techniques has different advantages and limitations. Spray coating techniques are often desired to allow the user to exert more direct control during the coating process or to allow coating in a touch-up manner. Conventional techniques that involve dipping heated parts into fluidized powder beds are the simplest form of powder coating since no electrostatic charging is necessary. Finally, electrostatic fluidized beds also involve relatively simple apparatus and may be desirable for parts having simple geometric shapes and structures or for continuous processes.
Air circulation systems are advantageously employed to draw excess powder and air out of the powder coating area of an operating hood or chamber associated with the particular coating method. Conventional techniques requiring heated parts and lightly fluidized beds of powder may not require an air circulation system, depending on the application. Spray coating techniques require a relatively high draw of air from the operating hood to collect the more significant excess amounts of powder sprayed from the gun. Electrostatic, fluidized bed techniques typically employ an air circulation system that more precisely draws lower amounts of air and powder from the operating hood to create the most effective cloud of charged powder. This cloud should not significantly exit the hood toward the operator but also cannot be suctioned out of the hood to such an extent that the charged powder cloud does not adequately form. In short, the electrostatically charged powder cloud should be sufficiently large and dense enough to facilitate quick, uniform and sufficient coating of the part.
Due, for example, to the varying requirements of different powder coating techniques, apparatus suitable for combined capabilities in these areas is problematic. It would be advantageous to provide powder coating apparatus which, for example, combines the benefits of fluidized bed-type apparatus and spray coating apparatus in a system which may be easily adapted for such different powder coating techniques.